1. Field of the Invention
The present invention relates to a new material useful as an ytterbium doped laser gain medium, and to laser systems using the same.
2. Description of Related Art
In recent years, high-power efficient semiconductor laser diodes and diode arrays have been used to great practical benefit as pump sources for solid state lasers. T. Y. Fan and R. L. Byer, "Diode Pumped Solid State Lasers", IEEE J. Quantum Electronics, 24, 895 (1988). The vast majority of this work has involved AlGaAs diode pump sources emitting in the 780-810 nm range to pump rare earth doped solid state laser materials including neodymium (J. Berger, D. F. Welch, D. R. Scifres, W. Streifer, and P. S. Cross, "High Power, High Efficiency Neodymium:Yttrium Aluminum Garnet Laser End Pumped by a Laser Diode Array", Appl. Phys, Lett., 51, 1212 (1987)), thulium (P. J. M. Suni and S. W. Henderson, "1-mJ/Pulse Tm:YAG Laser Pumped by a 3 Watt Diode Laser", Opt. Letters, 16, 817 (1991)), holmium (T. J. Kane and T. S. Kubo, "Diode-Pumped Single-Frequency Lasers and Q-switched Laser Using Tm:YAG and Tm,Ho:YAG", Proceedings on Advanced Solid State Lasers, 6, 136 (1990)), and erbium (D. K. Killinger, "Phonon-assisted Upconversion in 1.64 Micron Er:YAG Lasers", Digest of Technical Papers, Paper THJ4, p240 CLEO, 1987). In the past year or so, powerful and efficient InGaAs diode arrays emitting in the 900-1100 nm region have become available (D. P. Bour, P. Stabile, A. Rosen, W. Janton, L. Elbaum, and D. J. Holmes, "Two-Dimensional Array of High Power Strained Quantum Well Lasers with Wavelength of 0.95 Microns", Appl. Phys. Lett , 54, 2637 (1989)) opening up the possibility of pumping a solid state laser using ytterbium (Yb.sup.3+) as the laser active ion (W. F. Krupke and Lloyd L. Chase, "Ground-state depleted Solid-State-Lasers: Principles, Characteristics, and Scaling", Opt. and Quantum Electronics, 22, S1 (1990)) (as well as providing an alternative diode pump source for erbium lasers). Part of the current intense interest in diode pumped Yb lasers accrues to the fact that the energy-storage lifetime of the Yb.sup.3+ laser ions is typically in excess of a millisecond compared to a few hundred microseconds for the Nd.sup.3+ ion. This longer lifetime lowers the diode array power required to produce a given degree of energy storage in the laser material which results in a correspondingly lower cost for the pump array.
The first demonstration of a Yb solid state laser pumped by a narrowband source was published by Reinberg, et al.. in 1971 (A. R. Reinberg, L. A. Riseberg, R. M. Brown, R. W. Wacker, and W. C. Holton, "GaAs:Si LED Pumped Yb-Doped YAG Laser", Appl. Phys. Lett., 19, (1971)). Ytterbium doped yttrium aluminum garnet (Yb:YAG) was used as the solid state laser material; it was pumped at a wavelength near 940 nm using GaAs:Si light-emitting diodes (LEDs). Silicon doping was used to shift the pump light to about 940 nm from about 860 nm, typical of undoped GaAs LEDs. The Yb:YAG crystal was cooled to a temperature near 77.degree. K. to reduce resonant loss at the laser wavelength (1029 nm). The pump LEDs were also cooled to 77.degree. K. to increase LED power and efficiency. The need to operate this laser at cryogenic temperatures discouraged interest in this device and no practical or commercial use ensued.
With the recent emergence of highly practical diodes and diode arrays operating in the 900-1100 nm region (D. P. Bour, P. Stabile, A. Rosen, W. Janton, L. Elbaum, and D. J. Holmes, "Two-Dimensional Array of High Power Strained Quantum Well Lasers with Wavelength of 0.95 Microns", Appl. Phys. Lett., 54, 2637 (1989)), a renewal of interest in designing and building practical Yb solid state lasers occurred (W. F. Krupke and Lloyd L. Chase, supra). Additionally, Krupke and Chase described a method (so-called "ground state depletion mode") by which the deleterious effects of resonance loss which compromise the practical utility of the Yb:YAG laser could be substantially overcome at room temperature by properly designing and utilizing intense, narrowband pump sources, such as an appropriately intensified laser diode array (W. F. Krupke and Lloyd L. Chase, supra). In 1991, Lacovara, et al.. reported the demonstration of a room temperature Yb:YAG laser pumped with a broad-stripe InGaAs laser diode (P. Lacovara, H. K. Choi, C. A. Wang, R. L. Aggarwal, and T. Y. Fan, "Room-Temperature Diode-Pumped Yb:YAG Laser", Opt. Letters, 16, 1089 ( 1991)). Although operated at room temperature, the experimental arrangement did not incorporate a sufficiently intense diode pump, in the manner described by Krupke and Chase supra) which would have mitigated the deleterious effects due to resonance absorption loss, and which would have allowed a considerably higher performance (efficiency, etc.) to be achieved.
Based on our analyses, the Yb:YAG material gain system falls far short of possessing the spectroscopic/laser parameters which will permit the superior room temperature operation possible in a more ideal Yb-solid state laser gain medium. Specifically, the pump absorption cross-section of Yb:YAG (about 0.8.times.10.sup.-20 cm.sup.2) is quite low and necessitates using a Yb doping level greater than 10.sup.20 ions/cc in order to obtain efficient absorption of pump light. This relatively high doping level introduces a proportionally higher resonance absorption loss at the laser wavelength, increasing the threshold pump power and lowering laser efficiency (when operating only several times above threshold). The undesirably low pump absorption cross-section also results in a pump saturation flux of 22 kW/cm.sup.2, which severely taxes the degree of pump array intensification needed to realize the benefits of operating the Yb:YAG laser in the ground state depletion mode. Additionally, the relatively low stimulated emission cross-section of Yb:YAG (about 2.times.10.sup.-20 cm.sup.2) results in a relatively low small signal laser gain for a given pumping flux and a relatively high saturation fluence (about 9.5 J/cm.sup.2). The latter high value is uncomfortably close to the optical damage threshold of YAG (10-15 J/cm.sup.2), limiting the extraction and overall laser efficiency of pulsed Yb:YAG lasers.
In view of these spectroscopic/laser parameter deficiencies of Yb:YAG for an InGaAs diode pumped solid state laser, it is desirable to provide an ytterbium doped laser gain material more suitable for commercial applications. In our survey, the most widely used laser hosts (e.g., YLF, BaY.sub.2 F.sub.8, LaF.sub.3, etc.) were found not to offer significant improvements over Yb:YAG.